1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device and more particularly to a technique applicable effectively to a high-density integrated memory circuit formed by using a phase change material, a logic mixed memory having a memory circuit and a logic circuit on one and same semiconductor substrate, or a semiconductor integrated circuit device having an analog circuit. Particularly, the present invention is concerned with a non-volatile high-speed random access memory.
2. Description of Related Art
The growth of the non-volatile memory market under the demand for mobile devices typified by portable telephones is remarkable. Typical of the non-volatile memory is FLASH memory, which, however, is used as a programmable ROM because the speed thereof is essentially low. On the other hand, as memory for works, a high-speed RAM is needed and both FLASH and DRAM memories are mounted on portable devices. If an element having the features of those two memories can be implemented, the impact thereof is extremely strong because not only it becomes possible to integrate both FLASH and DRAM memories into a single chip but also all the semiconductor memories are substituted thereby.
One candidate for implementing the said element is a non-volatile memory using a phase change film, which is described in detail, for example, in Patent Document 1. The phase change memory is sometimes called PRAM, OUM, or ovonic unified memory. In this memory, a crystal phase of a memory element changes in accordance with Joule's heat which is generated by an electric current flowing in the memory element itself, whereby storage information is written. Chalcogenide is used as the material of the storage element. Chalcogenide is a material containing at least one of sulfur, selenium, and tellurium.
Next, an operation principle of the phase change memory will now be described briefly. For amorphization of a phase change portion, as shown in FIG. 2, such a reset pulse as causes heating to a temperature above the melting point Tm of the chalcogenide material and subsequent quenching is applied to the phase change portion. The melting point Tm is, for example, 600° C. A quenching time t1 is, for example, 2 nsec. To crystallize the phase change portion, the temperature of the phase change portion is maintained locally at a temperature in the range from a crystallization temperature Tc to the melting point Tm. At this time, the temperature is, for example, 400° C. The time required for the crystallization differs depending on the composition of chalcogenide material used, but is 50 nsec for example. Crystallizing the phase change portion of the phase change memory cell will hereinafter be referred to as setting operation, while amorphizing the phase change portion will hereinafter be referred to as resetting operation.
The phase change memory is characteristic in that the resistance value of the phase change portion changes two to three orders of magnitude in accordance with the crystal phase and that since this resistance value is used as a signal, a read signal is large and a sensing operation becomes easier, thus resulting in the speed of read being high.
The following patents are here mentioned as documents related to a phase change memory rewrite method. First, the thinking that a constant current pulse is used for the rewrite of a phase change memory is described in U.S. Pat. No. 5,883,827 and Japanese Unexamined Patent Publication No. 2002-541613. Further, in Japanese Patent Laid-open No. 2003-100085, it is described that a sample state is read before rewrite and then rewrite is performed with an appropriate electric signal based on the result of the read and that the wave form of the electric signal is adjusted and the fall time of the electric signal is delayed for allowing the phase change portion to be cooled slowly at the time of crystallization, thereby permitting crystallization to take place stably.
To make the phase change memory perform its setting operation, the temperature of the phase change portion must be held in a constant range from its crystallization temperature to its melting point. However, if a constant voltage source is used, as shown in FIG. 3, the phase change portion crystallizes into a state of low resistance and a large current flows in accordance with the Ohm's law, so that the phase change portion is overheated. As a result, the temperature of the phase change portion exceeds its melting point and thereafter the phase change portion is quenched, whereby there again arises the possibility of amorphization. Thus, with the constant voltage source, it has so far been difficult to set the phase change memory stably.